Recently, there has been rapidly developed a high level integration of control circuits in instruments for general use and industry.
When an extraordinarily high voltage (surge) is applied to electronic parts of semiconductors used in such control circuits, such parts may be destroyed. Accordingly, it becomes indispensable to take a countermeasure to meet the situation. As such a counterplan, varistors are generally employed. Among the rest, the zinc oxide varistor is widely available for the protection of various kinds of electronic instruments from unusually high voltages because the zinc oxide varistor has an excellent voltage nonlinearity and surge absorbing ability.
Hithertofore, there has been widely known a zinc oxide varistor provided with at least two electrodes on the surface of varistor element having zinc oxide as its main component. Further, materials for said electrodes, are disclosed in, for example, Patent Application Kokai SHO 62-290104 Official Gazette, etc., whose content is as follows:
Electrode material for a zinc oxide varistor was produced by the process wherein 5.0% by weight of a lead borosilicate glass powder composed of 50.0-85.0% by weight of PbO, 10.0-30.0% by weight of B.sub.2 O.sub.3 and 5.0-25.0% by weight of SiO.sub.2 was weighed out and then said powder together with Ag powder (65.0% by weight) were milled in a vehicle (30.0% by weight), in which ethyl cellulose was dissolved in butyl carbitol, to obtain a silver paste which is the electrode material.
And then said electrode material was applied onto a surface of a fired varistor element and heated to form an electrode.
Although the above zinc oxide varistor is excellent in voltage nonlinearity as mentioned above, further improvement in the voltage nonlinearity has been sought due to the desire of energy-saving and efficiency increase in the zinc oxide varistor.
Thus, responding to the above requirements, the present invention aims to provide a zinc oxide varistor further improved in voltage nonlinearity.